This invention resides in a press installation and in a method of operating such an installation. The invention relates particularly to large part-press installations, for example, in the form of press-lines are on multistage large part presses in the form of transfer presses.
Conventional presses include a mechanical press drive with an electric drive motor and a fly-wheel which serves as an energy storage device. A crank drive, a circular drive, an elbow drive or a similar drive converts the rotation of the fly-wheel into a back and forth movement of a plunger. The fly-wheel is so large dimensioned that its speed changes remains tolerable. As a result, it stores substantially more energy than is necessary for a single deformation procedure. At least, if the fly-wheel is firmly connected to the circular drive a correspondingly large amount of energy needs to be destroyed when the press is stopped.
Lately, more and more considerations are given to servo-presses which include servo-motors for driving the plunger and also auxiliary equipment. These servo-motors drive the plunger on the respective other equipment of the press without the use of a fly-wheel. Consequently, the respective servo-motor must provide the power peaks required by the plunger or other equipment.
German publication DE 10 2005 026 818 A1, for example, discloses a press with a drawing die which is provided with electric drives. The electric drives are connected to the drives for the main movement of the plunger and/or the auxiliary movements of workpiece transport elements by way of an, at least sequentially usable guide shaft and, on the other hand, by way of energy storage devices and/or energy exchange modules.
The connection between the main drives and the auxiliary drives and the drawing dies by way of guide shafts and energy exchange modules however is quite expensive.
Furthermore, German publication, DE 198 21 159 A discloses a deep-draw press whose plunger is driven by servo-motors via spindles. The drawing die is also driven by servo-motors via spindles. The servo-motors of the drawing die are also interconnected by electrical shafts. Both servo-motors are controllable by a program computer.
Servo-motor controlled machines cause varying loads on the power supply grid. This may cause excessive loads on the power supply grids. This may cause a problem occasionally already with a single machine, but is very problematic if several machines working in parallel are at peak loads all at the same time. In spite of efficient drive techniques, this may lead to energy losses which need to be avoided.
It is the object of the present invention to provide an improved servo-press installation which avoids excessive power supply peak requirements.